XIAP (X-linked Inhibitor of Apoptosis) is the most potent inhibitor of apoptosis among all the lAP proteins. XIAP is widely expressed in cancer cell lines and tumor tissues. High levels of XIAP protein have been associated with poor survival and lower remission rate in acute myelogenous leukemia patients and resistance of several types of cancer to current therapeutic agents. XIAP potently inhibits both intrinsic and extrinsic apoptosis pathways by binding and inhibiting the initiator and effector caspases, whose activity is crucial for the execution of apoptosis. Because XIAP blocks apoptosis at the down-stream effector phase, a point where multiple signaling pathways converge, strategies targeting XIAP may prove to be especially effective to overcome apoptosis resistance of cancer cells. Smac/ DIABLO was recently discovered as an endogenous antagonist of XIAP by binding to the BIR3 domain of XlAP. High-resolution 3D structures of Smac in complex with the XIAP BIR3 domain showed that the interaction between them is mediated by only four Smac residues and a small but well-defined binding groove in the XIP BIR3 domain, which is suitable for designing small molecule inhibitors. Recent studies using cell-permeable Smac-based peptides have shown that inhibition of XlAP by targeting its BIR3 domain represents a promising therapeutic strategy for the design of molecular-targeted new anti-cancer drugs to overcome apoptosis resistance of cancer cells. To date, non-peptide, cell-permeable, small molecule inhibitors of XIAP (Smac mimetics) have not been reported. In this grant, we propose to discover such novel small molecule inhibitors of XIAP through the following specific aims. Aim 1. Performance of structure-based database searching over 1,000,000 structurally diverse synthetic organic small molecules and natural products to identify promising potential XIAP inhibitors. Aim 2. Determination of the binding affinities of these potential XIAP inhibitors to the BIR3 domain of XIAP by the FP-based method followed by conclusive confirmation by the NMR-based methods. Specific Aim 3(a) Investigation of these small molecule inhibitors for their activity in inhibition of cell growth and induction of apoptosis in cancer cells with high levels of XIAP, and for their specificity using normal cells with low levels of XIAP. (b) Investigation of the molecular mechanism of action using Jurkat cells transfected with XlAP and vector control for several most potent small molecule inhibitors of XIAP.